The prior art has concerned itself, for many years, with the problem of reducing or eliminating the loss of water in galvanic cells using aqueous electrolyte and avoiding build up of excessive gas pressure in sealed cells. Hydrogen gas is evolved during charge or standby by several electrode materials as aluminum, magnesium, zinc, iron, lead, etc. The electrodes in general do not have the capability of recombining the hydrogen and the evolved gas is usually vented, causing water loss, or pressure build up in hermetically sealed cells. In sealed cells, depending on the amount of hydrogen present and the rate of generation, excessive gas pressure can build up causing rupture of the safety vent and loss of electrolyte -- resulting in cell failure and electrolyte leakage. It has previously been found that cells having a porous manganese dioxide cathode have the capability of recombining the hydrogen, provided catalytically active materials are applied to the cathode electrode.
Two approaches are often used in efforts to solve the problems. These are:
1. Catalytic recombination of hydrogen and oxygen inside or outside the battery; in the latter case, provisions are made for the return of the product water to the electrolyte chamber [U.S. Pat. No. 3,630,778 (1971), U.S. Pat. No. 3,598,653 (1971), U.S. Pat. No. 3,622,398 (1971), U.S. Pat. No. 3,701,691 (1972)]. PA0 2. 2 Use of an auxiliary (third) electrode as an overcharge recombination reactor as described in "Electrochem. Technol., 4, 383" (1966) by P. Ruetschi and J. B. Ockerman.
In fact, KORDESCH et al in U.S. Pat. No. 4,224,384 report excellent hydrogen gas absorption capability of dry MnO.sub.2 powder catalyzed with salts or oxides of platinum, palladium, ruthenium, rhodium, arsenic and lead. These materials, however, when employed in a wetted MnO.sub.2 matrix, did not show significant hydrogen recombination rates at near atmospheric pressures. It has now surprisingly been found that these materials exhibit hydrogen recombination properties provided the gas pressure is increased, for example, in the range of 5 to 15 psig or up to the relief pressure of the cell. Catalytically active carbon bonded with PTFE is also useful.
According to the invention there is provided a rechargeable electrochemical sealed cell having a cathode, a zinc anode, and an aqueous, alkaline electrolyte contacting the anode and the cathode in which cell hydrogen may evolve. The cathode comprises a metal oxide and auxiliary cathode material comprising a porous substrate and a catalyst for the absorption of pressurized hydrogen by the electrolyte, the auxiliary cathode material being located to be at least partially wetted by the electrolyte.
The substrate may be carbon, graphite or metal, and the catalyst may be carbon, catalytically active noble metals, salts and oxides of lead, nickel titanium lanthanum, chromium, tantalum and alloys thereof, and the noble metals or mixtures of carbon with the noble metals salts or oxides. The noble metals may be, for example, platinum, palladium, ruthenium, rhodium or silver.
The auxiliary cathode material may be provided either in admixture with the metal oxide cathode, suitably in a ratio of 30:70 respectively, or as a discrete auxiliary electrode in electronic contact with the metal oxide cathode.
When the auxiliary cathode material is provided as an auxiliary disorete electrode and the metal oxide cathode is cylindrically located about an anode core, then the auxiliary electrode is suitably an annulus or disk of similar diameter to the metal oxide electrode and located in electronic contact with it at one end of the anode.
The present invention may rovide economic and effective means of removing hydrogen oxygen gas in galvanic cells. Noble metals such as platinum, palladium, :rhodium, iridium, ruthenium, and osmium show high catalytic activity for hydrogen oxidation. in alkaline electrolytes, nickel and alloys of nickel with other metals e titanium and lanthanum) were found to be active catalysts. Gas diffusion electrodes applicable to the present invention are described in the co-pending United States Patent Disclosure "Metal and Metal Oxide Catalyzed Electrodes for Electrochemical Cells and Method of Making Same" by K. Tomancsohger and K. Kordesoh, Ser. No. 234,933, filed Aug. 22, 1988, and can be employed if higher recombination current densities are desired.
Embodiments of the invention will now be described by way of illustration with reference to the drawings in conjunction with the Examples, describing various electrodes of the invention and their operating characteristics.